1. Field of the Invention
This invention generally relates to a safety net arrangement for protecting workers and passersby from injury during construction of multi-floor buildings and, more particularly, for raising a fully deployed safety net without using guide rails attached to the building.
2. Description of Related Art
It was known from U.S. Pat. No. 3,949,834 to erect safety nets adjacent an outer face of a building under construction in order to catch workers and/or objects falling off a floor above the net, thereby protecting the fallen worker, workers on lower floors, as well as passersby at ground level from being injured. The outer edge of each net was supported at an outer end of a cantilevered support pole whose inner end was connected to a floor bracket bolted to a lower floor. The inner edge of each net was bolted to the next higher floor by eye-bolts or by cables tied to the nearest perimeter columns.
Despite its use in many buildings under construction the arrangement of the '834 patent was not easily movable from lower to higher floors as construction proceeded, since the net supports had to be unbolted and untied from lower floors prior to being moved, rebolted and re-tied on higher floors. Since the disassembled net exposed workers and passersby to potential injury from falling workers and/or objects, extra safety nets were erected while other safety nets were being moved--a procedure which was labor-intensive and costly.
U.S. Pat. No. 4,119,176 disclosed a safety net supported by Y-shaped support arms having rollers which rolled along and within a guide channel or track vertically attached adjacent the outer building face. Similarly, U.S. Pat. No. 4,732,234 disclosed a safety net supported by cantilevered struts connected to slide rails which slid along and within a guide channel or track vertically attached to the outer building face. Pending U.S. patent application Ser. No. 07/247,044 filed Sept. 20, 1988 also disclosed the movement of net support structures along and within guide channels or tracks vertically attached adjacent the outer building face.
Although generally satisfactory, the use of vertical tracks has not proven to be altogether practical. In a typical installation, each track has a predetermined length which extends over a distance of about two or three floors of the building under construction. In the construction of skyscrapers or buildings having, for example, 50 floors, it would be impractical to provide a single track whose length spans 50 or more floors. Hence, in practice, multiple tracks are used. Once a safety net was raised to the upper level of a first set of tracks bolted to the building, a second set of tracks was aligned with the first set and bolted to the building above the first set. This enabled the net to be raised along the second set of tracks. Prior to or as soon as the net was raised to the upper level of the second set of tracks, the first set of tracks was unbolted from the building and aligned with the second set of tracks, thereby enabling the net to be raised again. This approach, although workable, required additional tracks to be erected while others were being removed--a procedure which was labor-intensive and costly.
It was also known from U.S. Pat. No. 3,533,487 to utilize hoist cables for moving a scaffolding and safety net assembly upwardly and downwardly relative to a building. U.S. Pat. No. 4,129,127 also disclosed the sliding of a series of nets mounted in frames upwardly and downwardly along guide channels in upright beams anchored to a building under construction.
Still another drawback of known safety net arrangements resided in the risk that they would suddenly drop and fall either entirely or partway toward the ground. It was known to install locking pins to block the net supports to prevent such falls. It was also known to use frictional brakes to resist such falls. However, the failure to install locking pins and, in extreme cases, the shearing of such pins due to metal fatigue, compromised worker safety. Frictional contact brakes have not proven to be the most reliable way to prevent falls.